Optical systems include, for example, projection objectives in semiconductor lithography for fabricating finely patterned components. Optical systems of this type typically have a plurality of optical elements, which can be formed, for example, as lenses, mirrors or plane-parallel plates having a wide variety of reflection/refraction properties. A projection objective is commonly used to image a structure or a pattern of a mask (reticle) onto a light-sensitive substrate. The structure arranged in the object plane of the optical system is usually illuminated by an illumination source and its associated illumination optical assembly. The light transmitted through the structure can be directed through the optical system and expose the light-sensitive substrate arranged in the image plane of the optical system.
The imaging quality of an optical system is often impacted by imaging aberrations occurring in the optical system, such as, for example, areas resulting from aberrations. Such imaging aberrations can be thermally induced during the operation of the optical system by at least one optical element of the optical system being heated and altering the imaging properties of the optical system. Owing to the heating of the at least one optical element, an irreversible, radiation-dictated alteration of the material of the at least one optical element can occur. By way of example, an alteration of the density of the material (compaction) leads to a local change in the refractive index of the optical element. Furthermore, it is possible that a temporary, reversible alteration of the material of the optical element in the form of a change in refractive index or else a geometrical deformation can occur and can influence the imaging properties of the optical system. Depending on the illumination mode of the optical system, the heating of the at least one optical element can be rotationally symmetrical or non-rotationally symmetrical relative to the optical axis of the optical system. Non-rotationally symmetrical heating is brought about, for example, by dipole illumination produced by a mask or a grating in the illumination optical assembly.